CODEWITHSUNDEEP
crandom
pjreddie
pjreddie

Reputation: 410

Seeding random number generator for a batch of programs

So normally I use something like:

srand(time(0));

To get pseudo-randomness that changes with every program invocation. However, I'm now in a situation where I have a batch of programs that will all be starting at the same time and since time only changes every second, most of the time all of my programs start with the same seed.

What is a better strategy for seeding my RNG when I want a bunch of programs to start at once and all get different seeds?

Upvotes: 3

Views: 1021

Answers (3)

Tony Delroy
Tony Delroy

Reputation: 106236

You can combine time(0) with some other program-specific value. For example, XOR it with a decent hash of the program name (argv[0] would normally suffice), or even just (preferably a hash of) the process id (if they're launched on the same host, otherwise XOR further with a hash of the hostname or IP). You could even go so far as to use a hash of an UUID.

Note: just xoring with the process id is pretty weak - if the second ticks over while the processes are starting, it could easily be that the bits flipped in the time value match the bits that differ between two of your process ids, leaving the generated seed identical. That said, put in an amount of effort commensurate with how much you have reason to care....

Upvotes: 1

caskey
caskey

Reputation: 12695

Use srand(time(0) ^ getpid()) to permute your seed by a process-specific value. This will ensure different seeds for processes started within the same second. Take care to not use this for anything "important", e.g., cryptography or real money is involved.

The permutation is accomplished using the "exclusive-or" or XOR operator '^'. Because we know that two process running at the same time must necessarily have different process-ids, by xoring the response from time(0) with the current PID, we can get some assurance that two different processes won't have exactly the same seed.

Note that this is a very weak assurance as we are only twiddling a few bits. If the time were to increment by exactly one second and the process id were to increment by exactly one, in certain circumstances you would end up with identical seeds.

If you need truly distinct random number seeds, then you want to read 4 bytes from /dev/random and then use that as an integer to seed your RNG.

And again, PLEASE do not use this random number sequence for anything "important". And by important I mean anything more than a simple monte-carlo simulation or a game of rock-paper-scissors.

Upvotes: 7

Jonathan Leffler
Jonathan Leffler

Reputation: 754820

Maybe you need to use getpid() as well as some sub-second time (and perhaps whole second time too). Maybe make an MD5 hash of the values, and use that? Using getpid() guarantees that part of the data you're using will be unique to each process, but doesn't of itself give you very much randomness in your seed. Of course, using rand() is hardly guaranteed to be a good random number generator. You certainly can't use it for cryptography.

Upvotes: 0

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